Preparation of thermoset cycloolefin polymers via metathesis catalysts is a relatively recent development in the polymer art. Klosiewicz in U.S. Pat. Nos. 4,400,340 and 4,520,181 teaches preparation of such polymers from dicyclopentadiene and other similar cycloolefins via a two-stream reaction injection molding technique wherein a first stream, including the catalyst, and a second stream, including a catalyst activator, are combined in a mix head and immediately injected into a mold where polymerization and molding to a permanently fixed shape take place simultaneously.
In the presence of a metathesis catalyst system, polymerization takes place extremely rapidly even at low temperatures. In fact, polymerization occurs so rapidly that it is not unusual for the monomer to polymerize to a solid, immobile condition before the mixed streams can be transferred to the mold. To overcome this difficulty, Klosiewicz teaches the inclusion of a reaction rate moderator in the activator stream to delay the catalyst activation until the reaction mass is totally within the mold. The total time from mixing until polymerization is substantially complete is still just a matter of seconds.
In carrying out bulk molding by metathesis polymerization of crosslinking systems, two parameters are very important. When the liquid streams are first mixed, a short induction time is observed, following which polymerization begins and a rapid viscosity build-up takes place to a point at which the material becomes too viscous to be pumped to a mold. This time interval is referred to as the gel time. When the gel time is reached, the liquid must already be in the mold. Shortly following gel time a very rapid temperature increase is observed as the remainder of the polymerization and the bulk of the crosslinking take place. The time from mixing to attainment of 100.degree. C. is arbitrarily taken as the polymerization time (cure time) although the temperature rise continues to 175.degree. C. and higher. The time span between gel time and cure time is desirably very short so that mold cycle time can be maintained at an economical minimum. Ideally, the ratio between gel time and cure time should approach 1.0.
It is one of the advantages of this invention that, while the gel time is extended by a highly desirable amount to allow flexibility in molding operations, the ratio of gel time to cure time moves closer to 1.0 than is observed with the conventional system. That is to say, more time is allowed for mixing the solutions and transferring the mix to a mold, but the time interval between mixing and completion of the polymerization is not increased.
Due to the extremely rapid rate of reaction of cycloolefins, even in the presence of the rate-moderated catalyst, useful polymerization has heretofore been accomplished almost exclusively by the reaction injection molding (RIM) process using the two-stream process of Klosiewicz. Even in RIM processes, the short gelation times limit the application to relatively small items and to relatively non-detailed molds with a minimum of sharp corners which tend to trap pockets of air if the mold is filled too rapidly or if the viscosity of the polymerization mass builds up so rapidly that the gelled monomer does not flow easily into corners or around blocked-out sections. The polymerization mass cannot readily be employed in thermoset molding techniques such as pour, rotational and resin transfer (RTM) molding applications which require relatively long mold filling times.
It has been found possible (see Leach U.S. Pat. No. 4,458,037) to extend the gelation time to as much as ten minutes at room temperature by use of a dialkyl aluminum iodide activator moderated by di-n-butyl ether. When heated to 80.degree. C., this mixture polymerizes in about 15 seconds. This system is also unsatisfactory in procedures where filling of the mold takes place slowly since the mold temperature must be held low enough during the filling operation that the reaction mixture remains fluid until the mold is entirely filled and then raised to the reaction temperature. For commercially practical production rates to be attained, the differential between mold filling temperature and polymerization reaction temperature must be smaller than is possible using the catalyst of Leach.
Minchak, U.S. Pat. No. 4,426,502 teaches the use of a catalyst system based on an alkyl ammonium molybdate and an alkoxyalkyl aluminum halide. This system not only delays the initiation of the reaction for a significant time, but requires heat to be applied to the mold to trigger the polymerization.
Nelson, in U.S. Pat. No. 4,727,125 teaches moderating the reaction with certain specified amines which likewise give very substantial delays to allow for filling large molds and for using molding techniques other than RIM. As reported in Klosiewicz, U.S. Pat. No. 4,400,340 and described hereinabove, the metathesis polymerization of dicyclopentadiene (DCPD) is carried out by a technique wherein at least a first reactant stream containing DCPD and a metathesis catalyst component is combined in a mix head with a second reactant stream containing DCPD and a metathesis catalyst activator and a rate moderator. The mixture is then injected into a mold where the reaction to form a crosslinked dicyclopentadiene polymer in a predetermined, desired shape takes place. This same technique is taught by Minchak, Leach, and Nelson in their later patents, either with DCPD or other norbornene-type polycyclic cycloolefins.
In the preferred embodiments taught by Klosiewicz, the metathesis catalyst is tungsten hexachloride and the preferred catalyst activators are alkyl aluminum halides moderated by an alcohol, ester, ketone, nitrile or alcohol.
The tungsten or molybdenum catalyst is solubilized by complexing it with a phenolic compound so that a homogeneous catalyst/DCPD solution can be prepared. Also, in order to prevent premature ionic polymerization of the DCPD monomer in which the catalyst is to be dissolved, the catalyst component is stabilized by reacting it with a chelating agent or a Lewis base. Such chelants as acetylacetone, dibenzoyl methane, and alkylacetonates or Lewis bases such as benzonitrile or tetrahydrofuran can be employed as the stabilizer. The chelants and, particularly, acetylacetone (2,4-pentanedione), are preferred stabilizers. Stabilization of the catalyst prevents ionic polymerization, giving the solution an almost indefinite shelf life in the absence of any activating mechanism taking place.
For a full description of catalyst preparation, reference can be had to Martin U.S. Pat. No. 4,696,985.
While the technique of Klosiewicz works very well for a great number of applications and has been the technique employed for virtually all of the market penetration made to date by poly(dicyclopentadiene), there are applications in which a degree of modertion intermediate those of Klosiewicz at one extreme and Minchak, Leach, and Nelson at the other is desirable.